It's easy to remember: I don't have to go look it up in a slide I saw six months ago (like I did with the above definitions). I can remember it and explain it to someone off the top of my head, with little to no effort and with no appeal to authority. I don't even need to attribute it to James and Michael if it's a hallway conversation with a programmer or manager.

It's consistent with my experience: I find that this definition has applied to every project I've worked on. I've logged deviations from requirements that were closed as functions as designed. Those weren't bugs. I've logged inconsistencies in implementation that were closed as functions as designed. Those weren't bugs. I've even logged a security issue that allowed me to log into the production environment of a very large company without a user id or password. But that wasn't a bug either. None of those bugged the people who mattered. They only bugged me.

It's simple to explain: When I tell someone a bug is something that bugs somebody who matters, about the only follow up question I get is "Well, who matters?" Everyone seems to intuitively understand that this definition has a ring of truth. I find that it keeps me out of debates on word definitions and spares me from appealing to authorities that no one agrees on.